Properties of optical instruments. Visual optical systems part 2: focal visual instruments (microscope type)

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1 Properties of optical instruments Visual optical systems part 2: focal visual instruments (microscope type)

2 Examples of focal visual instruments magnifying glass Eyepieces Measuring microscopes from the students s lab Study and Research Microscopes

3 Microscope simplifié From the IO students lab: Basic microscope (viseur à frontale fixe) Interchangeable microscope objective Eyepiece (x10 standard) Fixed front focal length

4 Another instrument in IO students lab: Adjustable long distance viewer viewer (viseur dioptrique) Objective F F F oc Eyepiece Variable distance around the afocal position

5 Example of a study microscope

6 Example of a research microscope

7 Characteristics of visual instruments Size of image (magnification) Aperture Resolution Field of view Depth of field Telescope typ instr Object at infinity Angular magnification G Usually entrance pupil is primary lens or mirror Exit pupil adapted to eye pupil Resolution of the eye Diffraction, aberrations Internal lenses act as field stop, field lens in eyepiece adapted to eye field of view Connected with resolution by also accommodation of the eye Microscope type instr Object at finite distance Power P, magnifying power G Numerical aperture often entrance pupil is at infinity (telecentric stop) Transverse resolution, to compare to the naked eye similar similar

8 Size of Image for microscope type Microscope type : Object at finite distance, Image at infinity Object with size y, Image with angular size θ Power θ' = 1 Unit : = P y f ' diopters δ [m -1 ] y F H H F θ

9 Power of a compound microscope Power of the eyepiece: ' Peyepiece=θ y= 1 f ' eyepiece F y F θ in m -1 (diopters) H H Power of the compound microscope { objective + eyepiece } Objective Eyepiece y H F H θ F ep y F ep microscope objective y P = g. P eyepiece

10 Magnification for microscope type Comparison with angular size as seen with a naked eye y θ d=25cm y F H H F θ Magnification θ' G = = d = θ f ' Information given in the form (typical values): objective 10x, 25x for an eyepiece G = g. G objective 10x (g y ) associated with eyepiece 10x (P) P 4 microscope x100 for the microscope (up to 1000) y eyepiece

11 Different types of microscope objectives tube length 160mm or corrected at infinity (used with tube lens) Transmission (corrected for glass slide 0.17mm) or reflection mode Dry or water or oil immersion (lower( or higher NA)

12 Example of indications written on an objective : Microscope objective 60x, 1.40 Oil, /0.17 Eyepiece : 10x Magnification : 60 Numerical aperture in obj sp : 1,4 Designed for an object at infinity 0,17 mm coverslip over the object Magnification of eyepiece :10

13 Aperture of a microscope The aperture stop is usually in the second focal plane of the objective* Entrance pupil at infinity = Telecentric instrument in the object space A F obj Aperture stop PS : exit pupil (location of the eye pupil) *except for low magnification objectives

14 Entrance pupil at infinity = Telecentric instrument in the object space Why is it useful? B A F α α F The numerical aperture in the object space sin ( (α ) stays constant for any point in the field of view.

15 2nd advantage of the telecentric stop: it reduces the measurement or position error caused by a slight defocusing of the system scale in focus Telecentric stop scale in defocussed position error Principal ray F Stop at lens Principal or chief ray = oblique ray passing through the center of the aperture stop F

16 Exit pupil The numerical aperture in the image space of the objective is given by the Abbe Sine condition ny sin( α) = y 'sin( α') y ' α' α ' = nsin( α)/ gy A F obj Aperture stop Diameter of the exit pupil : Φ PS P.S. : exit pupil =2α' f ' ey

17 Example of indications written on an objective : Diameter and position of the exit pupil? Microscope objective 60x, 1.40 Oil, /0.17 Eyepiece : 10x

18 Resolution of the microscope Ability to separate 2 points should be limited by diffraction and not by the resolution of the eye Exit pupil smaller than 1 mm Better for large NA of the objective

19 Example 1 : Basic measuring microscope with objective 2.5x, 0.07, 160 /0.17 And eyepiece : 10x Resolution of the instrument?

20 Example 2 : Microscope objective 60x, 1.40 Oil, /0.17 Eyepiece : 10x Resolution? Limited by the eye or by diffraction?

21 Illumination of a microscope specimen Uniform illumination is important to distinguish low contrast object Resolution is a key property in a microscope Spatial coherence of the illumination (aperture of the condenser) has an influence on resolution and contrast For good control of aperture and field, the illumination configuration is done in the Köhler configuration

22 Köhler illumination Illuminating light path Image-forming light path Pu Dc α I DC DO C Pr Obj Oc Illuminateur Illuminator Condenseur Condensor Microscope Figure Principle 4: Principe of the de Köhler l'éclairage illumination de Köhler I : Optique Illuminator de l'illuminateur optics Pr: Préparation Specimen observée DC: Diaphragme Field stop de Champ Obj: Objectif Microscope du microscope objective Pu: Pupille Objective de l'objectif pupil DO: Diaphragme Aperture stop d'ouverture Oc: Eyepiece Oculaire Dc: Diaphragme Field aperture de champ of eyepiece de l'oc. C : Optique Condensor du condenseur optics Incoherent illumination Coherent illumination

23 Resolution and coherence of illumination Coherent illumination: small aperture stop for the illuminating path (1st focal plane of condenser), illumination with almost parallel beam Incoherent illumination: large aperture stop for the illuminating path, illumination with many different directions Better resolution limit for incoherent illumination Better contrast for coherent illumination

24 Influence of the coherence of the illumination on the quality of the images What happens in the plane of the pupil? S 1 S S 2 S 1 S S 2 S 1 S S 2 Modulation transfert function Coherent illumination Incoherent illumination Partially coherent illumination nsinα f grating < λ Spatial frequency f 2nsin λ α grating < ( f grating )

25 Field of view of a microscope What happens when we moves away from the axis? Objective Example of a basic microscope Eyepiece A A

26 Edge of bright field Max distance from axis when rays entering through the entrance pupil all exit through the instrument y BF θ BF y BF

27 Edge of total field Maximum distance from axis when NO MORE ray entering the entrance pupil exit through the instrument

28 Method for calculation object space Intermediate space Image space y BF θ BF y BF

29 Calculation in one of the spaces Through conjugation, it is equivalent to calculate the FOV in the object space (y BF, y TOT ) in the intermediate space (y BF, y TOT ) in the image space (θ BF, θ TOT ) Choose the space where the calculation is simpler! Then deduce the FOV in the other spaces using conjugation formulas Requires practising on problems

Properties of optical instruments

Properties of optical instruments Visual optical systems part 1: afocal systems (telescope type) A basic optical description of the eye Power: 60 diopters (at rest) Equivalent to a single spherical surface,